1. Field of the Invention
This invention relates to the detection of rare atomic species and the measurement of isotopic ratios with minimal backgrounds that are free from molecular interferences using accelerator mass spectrometry. Although limitations in scope are not intended, this invention has particular relevance to the fields of isotopic ratio measurements in geology, oceanography and nuclear weapons proliferation monitoring.
2. Description of Prior Art
During the last fifteen years, the sensitivity for detecting isotopes has been enhanced by many orders of magnitude using Tandem Accelerator Mass Spectrometry (AIMS). Descriptions of the technique include, for example, U.S. Pat. No. 4,037,100 to K. H. Purser and an article by D. Elmore and F. M. Phillips, "Accelerator Mass Spectrometry" in Science, Volume 236, p543 (1987). In the teachings of Purser described in the above U.S. Pat. No. 4,037,100, the atomic mass to be detected is accelerated as a negative ion beam and the charge exchange needed to fragment associated molecules takes place before the second acceleration stage. Thus, because the charge state tends to be high to guarantee fragmentations, the energy of the ions is increased considerably following fragmentation and before final analysis. Furthermore, to efficiently convert ions into the necessary high charge state, the tandem terminal voltage must be high leading to further increases in energy. The effect of this is that unnecessarily large systems are needed both for tandem acceleration and for the subsequent analyzers, with a corresponding increase in cost. While in a limited number of cases the extra energy is useful for isobar discrimination, heavy isobar pairs cannot be distinguished by energy loss or range measurements at realistic energies so that in many cases, the extra energy is a disadvantage rather than a gain. A second disadvantage is that not all molecular ambiguities are eliminated during a single fragmentation collision. Fragments having similar ratios of mass upon charge (M/q) and energy upon charge (E/q) will be transmitted without attenuation through all medium resolution electric deflection, magnetic deflection, crossed field and high frequency analyzers.
It is also the case that there are many examples where it is difficult to produce negative atomic ions of the wanted species. Thus, negative molecular ions that contain the atomic species of interest is often a convenient way of producing the particles needed for tandem acceleration. Negative ion oxides of beryllium, boron, aluminum, iron, and tantalum are therefore commonly used whenever these elements are to be analyzed. In some cases, such as nitrogen or magnesium, which do not form atomic negative ions, molecular ions are the only alternative. In practice, it may be desirable to use fluoride molecules for accelerating wanted elements. Fluorides are particularly useful molecules as fluorine is monoisotopic which reduces the number of isotopic mass combinations. It is known that uranium hexafluoride has a negative ion affinity of 5.6 electron volts which exceeds most atomic species by a factor of 2.
Furthermore, an important point to note is that the energy spectrum from sputtered molecular ions is significantly different from that of atomic ions. The high energy tags which are present in atomic ion spectra are reduced by approximately four orders of magnitude in the case of molecular ions. The presence of the above tail in an atomic ion spectrum creates difficulties whenever the mass of interest is greater than that of the more abundant isotope so that the use of molecules may be of great advantage.
3. Present Invention
The present invention comprehends an economical apparatus that carries out the task of efficiently determining isotopic ratios and elemental abundances when the concentration of the wanted elements are in the range of parts per billion to parts per trillion. The method to be described is generally applicable to a variety of sample compositions and to most elements in the periodic table. An important feature is that the primary ions from the source are not necessarily atomic species and when necessary can be molecules that include the isotopes of interest.
In the present invention, the concept of a tandem accelerator being used as a molecular fragmenter within its high voltage terminal as described in U.S. Pat. No. 4,037,100, has changed from that of a molecular disintegrator to that of a simple molecular accelerator. In this mode of operation, the necessary fragmentation to produce the wanted atomic mass is carried out by electron stripping using a foil or high pressure gas region at ground.
It is known that the majority of charge state 1.sup.+ molecular ions are stable and can be created by the removal of two electrons from the corresponding negative molecular ion. At the optimal terminal voltages used in this type of apparatus (1-3 million volts), the dominant charge state during terminal stripping will be 1.sup.+, 2.sup.+ and 3.sup.+. Yields of ions leaving the tandem in either 1.sup.+ or 2.sup.+ charge state can be dose to 50%, particularly if the stripper gas thickness is kept below equilibrium. Non-equilibrium production of charge state 1.sup.+ and 2.sup.+ molecular ions also assists in the subsequent analysis as energy losses due to straggling are reduced at the lower gas pressures. Using a small stripper thickness also reduces the small angle scattering which can be serious for heavy mass ions.
4. Discussion of Mass Ambiguities
Referring to FIG. 1, the energy, E, of atomic ions arriving at the tandem output, 15, is given by: EQU E=(Q+1).V.sub.T
Here,
Q is the charge state PA1 V.sub.T is the terminal potential of the tandem accelerator PA1 n=[(Mq/Q)-m]
For molecules, the energy, E.sub.m, of a molecular fragment or atomic ambiguity having mass, m, is given by: EQU E.sub.m =(Q+m/M).V.sub.T
To separate ions from these background particles on the basis of electrostatic deflection, the necessary resolution of an electrostatic deflector can be easily shown to be: EQU R.sub.el ={[(Q+1)/Q].times.qM}.times.1/n
where
Clearly, when Mq/Q=m, (n=0) the resolution, R.sub.el, needed to separate the molecular fragment of mass, m, and charge, q, from the wanted ions (M,Q) becomes infinite and those ions having mass, m, will be transmitted without attenuation to the detector. Clearly, ambiguities will be present whenever M/Q=m/q or when n is a small number 1, 2, 3, etc. These approximate m/q ambiguities are therefore of great importance especially in view of the Coulomb explosion which often occurs during the molecular fragmentation process adding substantial energy spreads to the beam in the laboratory frame of reference.
5. Second Charge Exchange
The above m/q ambiguities can be completely eliminated by a further charge exchange where the ions having the originally selected charge state, Q, are charge changed to a new charge state, Q', where Q' is not a multiple or sub-multiple of Q. For example, if Q is selected to be 3.sup.+, Q' might be 4.sup.+ or 5.sup.+. q which in the above example was 2.sup.+ or 1.sup.+ cannot take non-integral values and so the original m/q ambiguity particles can be eliminated by a following analysis stage with quite modest resolution.
Not only does the above change in charge state dramatically reduce the analyzer resolution needed, because the charge state is usually increased, the necessary electric and magnetic deflection capacity of succeeding analyzers is reduced.
6. Application
As an example of the use of such a charge exchange sequence, consider the detection of the long-lived radioisotope .sup.129 I. Using conventional AMS practice, it is necessary to accelerate .sup.129 I in the 5.sup.+ charge state. The reason for this is that .sup.129 I.sup.3+ cannot be used because of the m/M ambiguity from .sup.86 Sr.sup.2+ and .sup.43 Ca.sup.1+. The .sup.129 I.sup.4+ cannot be used because of .sup.97 Mo.sup.3+.
However, using .sup.129 I.sup.3+ from the tandem allows the conversion efficiency at the high voltage terminal to increase by a factor 8.1 for V.sub.T =1.05 MV and 4.45 for V.sub.T =2.95 MV. Due to the second charge change, a loss of efficiency is encountered which, because it is at equilibrium, is about a factor of one quarter in both cases, (although this can be improved because the charge change can be non-equilibrium). Overall, however, the transmission efficiency through the spectrometer is comparable for the classical AMS sequence and the mode of operation described here. The advantage is greatly reduced requirements for magnetic and electric analyzers reduces the cost and space needed.
A second example is that when ratios of fairly rare isotopes and m/q ambiguities from intense components such as meteoritic background or silicon wafers are encountered, the elimination of these ambiguities is critical. For example, iron isotope ratios in silicon .sup.54 Fe.sup.+3 will not be measured easily because of .sup.18 O.sup.1+ and .sup.19 F.sup.1+, .sup.38 A.sup.2+, .sup.56 Fe.sup.4+ is not practical because of .sup.28 Si.sup.2+. .sup.57 Fe.sup.+3 could have .sup.19 F.sup.+1 or .sup.38 Ar.sup.+2, etc., thus, 3.sup.+ and 4.sup.+ cannot be used for all isotopes. However, with charge changing, this situation is dramatically improved and the n=0, 1, 2, 3, etc. Interferences are eliminated.